The present invention relates to horology and more particularly to a method for adjusting (trimming) the effective frequency of an oscillator.
In recent years great interest has been shown in using piezoelectric crystal oscillators as highly accurate time bases in horological instruments.
However, it is difficult and, therefore, expensive to produce a small piezoelectric crystal having an exact predetermined frequency. For example, an error of only 0.01% (one part in ten thousand) in the frequency results in an error of about ten seconds a day or five minutes a month, which is unacceptable. This difficulty increases when the problems of mass production are considered.
The prior art typically discloses two types of correcting devices.
The first type corrects the frequency of the oscillator itself by means of a trimmer capacitor in series with the quartz crystal.
The second known type correcting device uses digital correction which is carried out by suppressing, at the input of the frequency divider, a certain number of pulses provided by an oscillator whose frequency is higher than a nominal value.
According to the present invention, a method and apparatus of the second type is provided.
An example of the second type of device is seen in U.S. Pat. No. 3,931,703 issued Jan. 13, 1976 to Igor Scherier et al. This patent discloses a device for correcting running of an electronic watch comprising two contactors acting on respective bistable circuits in turn acting on a combinatory circuit controlling a counter which provides binary correction information to an inhibitor circuit. The two push-buttons (contactors) are depressed, depending on the direction of correction required to advance or retard the displayed time. Thus, this device includes an operator programmable memory for manual time correction.
Another example of the second type of correction device is seen in U.S. Pat. No. 4,055,945 issued Nov. 1, 1977 to Jack Schwarzschild et al. This patent discloses a frequency adjustment device which is also programed by means of a switch arrangement that includes multi-position detent pin switches.
Other prior art patents which disclose frequency adjustment means are U.S. Pat. Nos. 3,540,207 issued Nov. 27, 1970 to E. R. Keeler; 3,914,706 issued Oct. 22, 1975 to Walter Hammer et al; 3,895,486 issued July 22, 1975 to Walter Hammer et al; 3,833,999 issued Sept. 10, 1974 to Irvin Budych et al; 4,004,447 issued Feb. 22, 1977 to Hans-Joachim Wolf Freinstein et al; and 4,093,877 issued June 6, 1978 to Jerald W. Vannier et al.
In contrast to the prior art, the present invention provides a method and apparatus which enables permanent frequency trim of an oscillator frequency, includes a nonvolatile programmable read-only-memory circuit (hereinafter ROM) programmed by electromagnetically detecting the uncorrected oscillator frequency and writing a correction (inhibit pulse) number permanently into the ROM, is adapted for mass manufacture and involves a minimum of process steps and associated parts/equipment.